The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue, and more particularly, to an EP apparatus and method for assessing the adequacy of contact between an ablation electrode and biological tissue. The invention also relates to an apparatus and method for providing energy to biological tissue while simultaneously monitoring the electrical activity within the tissue.
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. In this therapy, transmural ablation lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C.
In order to produce effective transmural lesions it is necessary to ensure that the electrodes are in intimate contact with the tissue. Positioning of the electrodes is typically done visually under fluoroscopy imaging and is thus largely a function of a physician""s training and experience. Assessment of adequate electrode/tissue contact is somewhat of an art and verification, at present, is typically inferred through comparison of pre- and post-ablation electrocardiogram (xe2x80x9cECGxe2x80x9d) analysis.
The use of impedance as an indication of electrode/tissue contact has been reported in the treatment of focal arrhythmias, such as ventricular tachyarrhythmia. In these procedures, a catheter with a single combination ablation/impedance-measuring tip electrode is inserted into the local blood pool within the heart and an impedance measurement is taken. The tip electrode is then placed at an ablation location and, so as to push the tip electrode deep into the cardiac tissue, force is applied along the axis of the catheter. An impedance measurement is then taken and compared to the impedance of the blood pool. This subsequent impedance measurement is referred to as a xe2x80x9ccontact-assessmentxe2x80x9d impedance. A significant increase in the contact-assessment impedance relative the blood-pool impedance serves as an indication that the tip electrode is in contact with cardiac tissue.
In this procedure a significant increase in impedance is noted due to the fact that the tip electrode is pushed deep into the cardiac tissue and is thus largely surrounded by tissue, as opposed to blood. While this electrode/tissue contact assessment technique is effective for the treatment of focal arrhythmias, it is less effective for the treatment of non-focal arrhythmias, such as atrial fibrillation. Ablation therapy for atrial fibrillation often involves the formation of transmural linear lesions. In this form of ablation therapy a linear array of band electrodes is placed against the atrial wall. While the band electrodes are held against the tissue with some degree of force, a portion of the band electrodes is likely to remain in the blood pool. The presence of blood against a portion of the band electrode affects the impedance measurement and reduces the significance of the difference between the blood-pool impedance and the contact-assessment impedance. Thus, the above-described electrode/tissue contact assessment technique that relies on the use of a tip electrode forced into the tissue is ineffective for linear ablation therapy. This known technique is further ineffective for linear ablation because it does not account for fluctuations in impedance measurements which may occur due to movement of electrodes caused by respiration and heart contractions.
As previously mentioned, in present ablation procedures, once ablation therapy is completed, the effectiveness of the therapy is verified through electrocardiogram (xe2x80x9cECGxe2x80x9d) analysis. Ablation therapy is completed upon the application of ablation energy for a prespecified time period. Once ablation therapy is completed, the ablation electrode is disconnected from the ablation energy source and is reconnected to an ECG amplifier/recorder. The ECG amplifier/recorder collects electrical data from the heart through the ablation electrode. The ECG amplifier/recorder analyzes the electrical data and produces signals indicative of the electrical activity through the heart tissue and particularly the ablated tissue. This present technique of assessing the effectiveness of ablation is inconvenient in that it requires ablation therapy be completed prior to assessing the ablation results and further requires physical switching from the ablation source to the ECG amplifier/recorder.
Hence, those skilled in the art have recognized a need for an RF ablation apparatus and method for assessing the adequacy of the contact between biological tissue and an ablation electrode positioned against the tissue but not necessarily completely surrounded by tissue. The need for an apparatus and a method for providing ablation energy to biological tissue while simultaneously monitoring the electrical activity within the tissue has also been recognized. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to an apparatus and method for assessing the adequacy of contact between an ablation electrode and biological tissue. The invention is also directed to an apparatus and method for providing energy to biological tissue while simultaneously monitoring the electrical activity within the tissue.
In a first aspect, the invention relates to a method of assessing the adequacy of contact between an ablation electrode carried by an electrode device and biological tissue within a biological organ having biological fluid therein. The method includes the steps of positioning the ablation electrode in the biological fluid; positioning a reference electrode a distance from the first electrode and the biological tissue and obtaining a reference impedance value by measuring the impedance between the ablation electrode and the reference electrode. The method further includes the steps of moving the ablation electrode to a position xe2x80x9cproximalxe2x80x9d, i. e., near or next to, but not necessarily in contact with, the biological tissue; obtaining an assessment impedance value by measuring the impedance between the ablation electrode and the reference electrode; analyzing the assessment impedance and the reference impedance; and indicating the state of electrode/tissue contact.
In a more detailed aspect, the step of analyzing the assessment impedance and the reference impedance includes the step of calculating the percentage difference between the two impedances. Furthermore, the step of indicating the state of electrode/tissue contact includes the steps of, when the percentage difference is approximately 10% or more, indicating substantially complete electrode/tissue contact; when the percentage difference is in the approximate range between 5% and 10%, indicating partial electrode/tissue contact; and when the percentage difference is less than approximately 5%, indicating no electrode/tissue contact. In another facet, the reference impedance value is the average of a plurality of reference impedance values obtained during a given time period. In yet another facet, the assessment impedance value is the average value of a plurality of assessment impedance values obtained during a given time period. In still another detailed facet, the method further includes the step of, prior to obtaining an assessment impedance value, positioning an electrical insulator relative the ablation electrode so that when the ablation electrode is proximal the biological tissue the electrode is interposed between the electrical insulator and the tissue.
In a second facet, the invention relates to a method of assessing the adequacy of contact between a plurality of ablation electrodes carried by an electrode device and biological tissue within a biological organ having biological fluid therein. The method includes the steps of obtaining a reference impedance value by positioning the plurality of ablation electrodes in the biological fluid; positioning a first reference electrode a distance from the plurality of ablation electrodes and the biological tissue; and measuring the impedance between at least one of the ablation electrodes and the reference electrode. The method also includes the step of moving the plurality of ablation electrodes to a position proximal the biological tissue; and for each ablation electrode, obtaining an assessment impedance value by positioning a second reference electrode a distance from the ablation electrode and the biological tissue and measuring the impedance between the ablation electrode and the reference electrode; analyzing the assessment impedance and the reference impedance; and indicating the state of electrode/tissue contact.
In a third aspect, the invention relates to a method of assessing the adequacy of contact between a plurality of ablation electrodes carried by an electrode device and biological tissue within a biological organ having biological fluid therein. The method includes the steps of obtaining a reference impedance value by positioning the plurality of ablation electrodes in the biological fluid; positioning a first reference electrode a distance from the plurality of ablation electrodes and the biological tissue; and measuring the impedance between at least one of the ablation electrodes and the reference electrode. The method further includes the step of moving the plurality of ablation electrodes to a position proximal the biological tissue; obtaining an assessment impedance value by measuring the impedance between selected pairs of ablation electrodes; analyzing the assessment impedance and the reference impedance; and indicating the state of electrode/tissue contact.
In a fourth facet, the invention relates to a method of assessing the adequacy of contact between an ablation electrode and biological tissue within a moving biological organ having biological fluid therein. The method includes the steps of positioning the ablation electrode proximal the biological tissue; positioning a reference electrode a distance from the ablation electrode and applying a signal to the ablation electrode during a time period sufficient to include several movements of the organ. The method further includes the steps of obtaining a sequence of impedance values by periodically measuring the impedance between the ablation electrode and the reference electrode during the time period and monitoring the sequence of impedance values for variations indicative of electrode/tissue contact.
In a more detailed aspect, the step of monitoring the sequence of impedance values for variations indicative of electrode/tissue contact includes the steps of obtaining an average impedance value based on a plurality of the impedance values, calculating the standard deviation of the impedance values relative the average impedance and calculating a xe2x80x9cdeviation percentage.xe2x80x9d The deviation percentage is the standard deviation over the average impedance, represented as a percentage. Further included are the steps of, when the deviation percentage is at least approximately 2%, indicating substantially complete electrode/tissue contact; when the deviation percentage is in the approximate range between 1% and 2%, indicating partial electrode/tissue contact; and when the deviation percentage is less than approximately 1%, indicating no electrode/tissue contact.
In a fifth facet, the invention relates to a method of assessing the adequacy of contact between an ablation electrode and biological tissue within a biological organ having biological fluid therein. The method includes the steps of positioning the ablation electrode proximal the biological tissue; positioning a reference electrode a distance from the ablation electrode; measuring the impedance between the ablation electrode and the reference electrode at a first frequency and measuring the impedance between the ablation electrode and the reference electrode at a second frequency. The method further includes the steps of analyzing the first-frequency impedance and the second-frequency impedance and indicating the state of electrode/tissue contact.
In a more detailed facet, the step of analyzing the first-frequency impedance and the second-frequency impedance includes the step of calculating the percentage difference between the two impedances. Furthermore, the step of indicating the state of electrode/tissue contact includes the steps of, when the percentage difference is approximately 10% or more, indicating substantially complete electrode/tissue contact; when the percentage difference is in the approximate range between 5% and 10%, indicating partial electrode/tissue contact; and when the percentage difference is less than approximately 5%, indicating no electrode/tissue contact. In another facet, the step of analyzing the first-frequency impedance and the second-frequency impedance includes the steps of calculating the ratio of the two impedances and comparing the ratio to a known value. Also, the step of indicating the state of electrode/tissue contact includes the steps of, when the ratio is approximately equal to the known value, indicating no electrode/tissue contact; when the ratio deviates from the known value by an amount in the approximate range between xc2x10.1 to xc2x10.15, indicating at least partial electrode/tissue contact; and when the ratio deviates from the known value by an amount approximately greater than xc2x10.15, indicating substantially complete electrode/tissue contact.
In a sixth aspect, the invention relates to an apparatus for assessing the adequacy of contact between an ablation electrode carried by an electrode device and biological tissue within a biological organ having biological fluid therein. The apparatus includes a signal generating device providing as output a drive signal to the ablation electrode and a reference potential and a reference electrode spaced from the ablation electrode and responsive to the reference potential. The apparatus further includes an impedance measurement device for providing a reference impedance indicative of the impedance between the ablation electrode and the reference electrode when the ablation electrode is positioned in the biological fluid and for providing an assessment impedance indicative of the impedance between the ablation electrode and the reference electrode when the ablation electrode is positioned proximal the biological tissue; and a processor responsive to the reference and assessment impedance signals for analyzing the impedance signals and indicating the state of electrode/tissue contact.
In a seventh facet, the invention relates to an apparatus for assessing the adequacy of contact between an ablation electrode carried by an electrode device and biological tissue within a biological organ having biological fluid therein. The apparatus includes a signal generating device providing as output a drive signal to the ablation electrode and a reference signal and a reference electrode spaced from the ablation electrode and responsive to the reference signal. The apparatus further includes an impedance measurement device for providing a sequence of assessment impedance values indicative of the impedance between the ablation electrode and the reference electrode and a processor responsive to the sequence of assessment impedance signals for monitoring the sequence of impedance values for variations indicative of electrode/tissue contact.
In an eighth aspect, the invention relates to an apparatus for assessing the adequacy of contact between an ablation electrode carried by an electrode device and biological tissue within a biological organ having biological fluid therein. The apparatus includes a signal generating device providing as output a reference signal and for a first time period, a first drive signal to the ablation electrode, the first drive signal having a first amplitude and first frequency, the signal generating device also providing as output for a second time period, a second drive signal to the ablation electrode, the second drive signal having a second amplitude and a second frequency. The apparatus further includes a reference electrode spaced from the first electrode and responsive to the reference signal; an impedance measurement device for producing as output a first assessment impedance signal indicative of the impedance between the ablation electrode and reference electrode during the first time period and a second assessment impedance signal indicative of the impedance between the first and second electrodes during the second time period and a processor responsive to the first and second assessment impedance signals for comparing the impedances to a predetermined value indicative of electrode/tissue contact.
In a ninth facet, the invention relates to a method of providing ablation energy to biological tissue through an electrode device having at least one electrode while monitoring the electrical activity of the tissue. The method includes the steps of positioning the at least one electrode proximal the tissue; applying ablation power to the at least one electrode through a first lead, the ablation power comprising a high frequency component and receiving, from the electrode and through the first lead, a feedback signal indicative of the electrical activity in the tissue. The method also includes the steps of filtering the feedback signal to remove any high frequency components and providing the filtered feedback signal to an instrument through a second lead.
In a tenth aspect, the invention relates to an apparatus for providing ablation power to biological tissue through an electrode device having at least one electrode positioned proximal the tissue. The apparatus includes a generator producing ablation power having a high-frequency component; a high-frequency filter; a first lead presenting the ablation power to the at least one electrode and the filter, the first lead further presenting a feedback signal from the electrode to the filter and a second lead presenting a filter output to an instrument.
In an eleventh facet, the invention relates to an apparatus including a generator producing a plurality of ablation power signals, each having a high frequency component; a plurality of high-frequency filters; an electrode device having a plurality of electrodes; a plurality of first leads, each presenting one of the ablation power signals to one of the electrodes and one of the filters, the first lead further presenting a feedback signal from the electrode to the filter; and a plurality of second leads, each presenting a filter output to an instrument.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.